Study on bubble behavior of inert gases at entrance nozzle in sodium-cooled fast reactor

Bubbles of inert gases exist in a primary coolant system of sodium-cooled fast reactor. Those bubbles may cause increase of reactivity in the core and cavitation erosion and so on. Therefore it is necessary from the viewpoint of reactor safety to understand the dynamic behavior of the bubbles. In this study, the model of gas transport at the entrance nozzle has been developed using a non - dimensional correlation based on a dynamic bubbles behavior. For this purpose, a three-dimensional analysis of dynamic bubbles behavior has been carried out by coupling one-way bubble tracking method with the multi-dimensional CFD tool, FLUENT. Then fractions of the bubbles out flowed through the entrance nozzle (f_in), accumulating under the core support plate (f_res) and dissolving in the liquid sodium (f_dis) are calculated. As a result, it has been demonstrated that f_in and f_res are influenced by the sodium flow field, bubble radius and the shape of the entrance nozzle, whereas f_dis is negligibly minimal. The authors have also developed a correlation for the gas behavior based on the dimensionless number which corresponds to the geometric, the buoyancy and the drag force effects acting on the bubble. (author)